As described in the previous annual report, a monoclonal anti-d antibody is capable of neutralizing the infectivity of hepatitis B virus (HBV) of the adw subtype in a susceptible chimpanzee. We have since inoculated a control chimpanzee with a mixture of this monoclonal anti-d antibody and HBV of the ayw subtype (1000 CID50). The levels of HBV serological markers (Hbsag, anti-HBs and anti-HBc) and ALT liver enzyme were monitored with weekly blood specimens. Hbsag was detected beginning four weeks after inoculation, and the ALT level exceeded 2.5 times baseline at 13 weeks, peaking at 15 weeks. Thus, this monoclonal anti-d antibody is specific to ad but not to ay subtype and the epitopes recognized by this anti-d may be a potential HBV vaccine.